Mixed-mode ?-? Fatigue Crack Propagation Theory And Test Method Based On Low Cycle Fatigue Properties

The Manson-Coffin law of uniaxial sample reflects the low cycle fatigue(LCF)properties of the material's representative volume element(RVE).The Manson-Coffin law and the fatigue crack propagation(FCP)rate are important index of mechanical properties for characterizing the fatigue failure of materials,they are the important basis for structural integrity assessment of key projects such as nuclear reactor engineering,chemical engineering,aeronautics and astronautics,and high-speed railway,etc.Researches in the past30 years indicate that FCP rate of mode-? flawed component is strongly correlated to LCF properties of RVE of material.For the mixed-mode ?-? crack,due to the more complex stress-strain field of the crack tip,whether and how there is any correlation between the FCP rate and the LCF properties of the material's RVE has not been reported in the literature.The stress intensity factor(SIF)and the J-integral of flawed components associated with geometrical dimensions,materials properties,and loads are the fracture parameters that govern crack propagation.Analytical solution of J-integral for mixed-mode ?-? cracks is still a problem in fracture mechanics.The theoretical formula and effective compliance test method for the FCP testing of mixed-mode ?-? crack(FCP_?-?)are still missing.This thesis is now devoted to finding the correlation between Manson-Coffin law and FCP_?-? rate,developing the semi-analytical model for J_e-integral computation of mixed-mode ?-? crack based on energy equivalence principle,and the compliance-based test method for FCP_?-?.The main contribution of this work includes:1.The relationship between the critical equivalent plastic-strain energy density(PED_c)and the lifetime of the uniaxial specimen can effectively characterize the critical failure characteristics of the material's RVE under cyclic plastic strain.The PED_c of the material's RVE is assumed to be equivalent to the critical mean equivalent plastic-strain energy density(PED_m,c)of the material's RVE in the effective cyclic plastic zone ahead of crack tip of mixed-mode ?-? crack under small scale yielding condition.By combining with the cyclic stress-strain field in the effective cyclic plastic zone ahead of crack tip,and considering the crack closure effect,the FCP rate model of mixed-mode ?-? crack due to LCF properties based on PED_c(FCPM_?-?-LCF-PED_c)is proposed.Based on this model,the fatigue life of mixed-mode ?-? flawed component can be theoretically predicted by using the parameters of Manson-Coffin model.2.The critical linear damage accumulation(LDA_c)of the material's RVE under uniaxial LCF condition is supposed to be equivalent to the critical mean linear damage accumulation(LDA_m,c)of the material's RVE in the effective cyclic plastic zone ahead of crack tip of mixed-mode ?-? crack under small scale yielding condition.By combining with cyclic stress-strain field in the effective cyclic plastic zone ahead of crack tip,and considering the crack closure effect,the FCP rate model of mixed-mode ?-? crack due to LCF properties based on LDA_c(FCPM_?-?-LCF-LDA_c)is proposed.The fatigue life of mixed-mode ?-? flawed component can also be theoretically predicted by using the parameters of Manson-Coffin model.3.For mixed-mode ?-? flawed components,based on the mean-value energy equivalence principle that the average strain energy density of the deforming domain is equivalent to the RVE strain energy density of the energy median point according to von Mises yield criterion,a theoretical method to describe the relationship between material's elastic parameters,geometrical dimensions,load(or displacement)and energy is proposed.The J_e0-CTS model and J_e0-Arcan model have been established for the compact tensile shear(CTS)specimen and shear butterfly specimen(Arcan specimen)with initial horizontal crack.For the CTS specimen and Arcan specimen,when the ratio of initial horizontal crack length a₀ to specimen width W satisfies a₀/W?[0.26,0.74]and a₀/W?[0.16,0.65],respectively,the loading angle?satisfies??[0,?/2],the predicting J_e-integral values by the J_e0-CTS model and J_e0-Arcan model match well with finite element analysis(FEA)results,and all the relative errors are lower than 1%.4.Based on the maximum circumferential stress criterion,the models of J_e-integral and compliance for the CTS specimens with horizontal cracks deflecting and propagating(flat-folding propagation)under different loading angles,geometries and materials are proposed.Combined with innovative design of the fixture of CTS specimen used for FCP_?-?testing,a new compliance-based test method for FCP_?-?(CTM-FCP_?-?)is proposed.For the CTS specimens whose initial horizontal crack length a₀ and the subsequent inclined crack length a satisfy a₀/W?[0.36,0.64]and a/W?[0,0.2],when the loading angle?satisfies??[0,?/2),the predicted results of J_e-integral and the compliance are in good agreement with FEA results,and all the relative errors are less than 1%.5.A series of FCP tests for 30Cr2Ni4Mo V rotor steel were carried out by using the CTM-FCP_?-? method.The experimental results of the FCP rate under different loading angles and different initial crack lengths were obtained.The predicted results of FCP rate based on the two kinds of theoretical models are in good agreement with experimental results.